The Radio Link Control (RLC) protocol has been standardized in the 3GPP specification TS. 25.322. There have been established three types of RLC entities with a different working methodology. The three types are: Transparent Mode (TM), Unacknowledged Mode (UM), and Acknowledged Mode (AM) RLC entities. The two first entities do not handle retransmissions in contrast with the third, the Acknowledged Mode (AM) RLC, which deploys a window based system to retransmit lost packets.
The AM RLC is window based and transmissions from a sender to a receiver are executed for as long as a sliding window is not depleted. The sliding window has a fixed size and it indicates the quantity of Protocol Data Units PDUs that are possible for a sender to keep and still continue sending data to the receiver without receiving an acknowledge.
When the quantity of PDUs that are possible for the sender to keep has been reached, the sender has to stop the sending of new PDUs until acknowledgements for previous PDUs are received. In AM RLC the acknowledgements are sent by the receiver to the sender in the form of status reports. The status reports can be viewed as a list of acknowledgments that are transmitted from the receiver to the sender in response to some predefined and configured trigger events. If the sender receives a status report, any and all negatively acknowledged PDUs have to be retransmitted. The triggers for sending a status report are listed in 3GPP TS 25.322 Radio Link Control (RLC) protocol specification and include PDUs with the poll bit set, which indicates to the receiver that the status report has to be delivered to the sender. Also the detection of missing PDUs and a periodical timer are triggers for status reports.
The status reports are sent after they have been triggered if the status prohibit timer is not running. The status prohibit timer is a configurable timer that prevents the receiver to send status reports during a specific amount of time. The purpose of the status prohibit timer is to avoid too frequent status reporting. If there are several status reports within a RLC round trip time (RTT), this may lead to unnecessary retransmissions, and poor protocol performance.
The status prohibit timer is set after one status report is sent and the receiver can not send a new status report until the timer expires. If during this time a status report is triggered, for example by any of the previously mentioned functions, it is postponed until the status prohibit timer has expired. The status reports are not retransmitted. Also the status reports do not have a sequence number and the sliding window is updated with the last received report.
The High Speed Downlink Shared Channel (HS-DSCH) is a high speed packet data channel defined in 3GPP Rel-5. The Enhanced Dedicated Channel (E-DCH) is the uplink counterpart specified in 3GPP Rel-6. Both HS-DSCH and E-DCH employ Hybrid Automatic Repeat Request (HARQ) with soft combining. The introduction of retransmission in HARQ level brought a need for reordering queues in the NodeB. The standard 3GPP TS 25.321 Medium Access Control (MAC) protocol specification specifies in-sequence delivery to the upper layers, and since a packet transmitted using HARQ can be received after several attempts of transmission causing an out-of-sequence delivery, so called reordering queues are implemented.
In existing solutions the peak performance of HS-DSCH in the downlink and E-DCH in the uplink is to a large degree limited by the RLC sliding window stalling when RLC AM is used. The RLC window stalls when the status reports have not been able to slide the window because they are delayed by HARQ retransmissions, or not received at all, for example due to HARQ failure.
Hence, there exist a need for a method and a system that is able to increase the performance in a system using RLC AM.